(1) Field of the Invention
This invention relates to an optical fiber, especially to an optical fiber for propagating an optical wave using a linearly polarized optical wave, and also to a method of producing such an optical fiber.
(2) Description of the Prior Art
Along with the progress in development of optical fibers, steady development has been made to realize optical circuits such as an isolator, a switching circuit mixer and the like by means of an optical integrated circuit. A waveguide structure used in the optical integrated circuit is fundamentally of a slab type. To realize the switching circuit by the optical integrated circuit, the waveguide must be formed in the slab structure, too, for the necessity of polarizing the light. It is desired in the art to effectively combine such an integrated circuit with other optical devices by use of the optical fiber, but in such a case, the optical fiber must be capable of maintaining the polarization plane in a specific direction.
Further, there have been made various proposals to carry out a variety of measurements by making use of the polarization plane of the light. To realize such measurement, a critical problem to be solved is to transmit the light while maintaining its linear polarization. It would be desirable both from the viewpoints of transmission efficiency and production technique if transmission of the light could be made while maintaining the linear polarization by use of a circular optical waveguide or optical fiber that has now reached the stage of practical use.
To meet the abovementioned requirement, there has been proposed a circular waveguide which forms an optical fiber using a fibrous single crystal consisting of a specific, optically anisotropic material, and of which the refractive index distribution in its section enables single mode transmission but allows the light to leak outside thereby to prohibit the transmission in the orthogonal direction (Japanese Patent Laid-Open No. 60534/1676 entitled "Optical Waveguide", laid-open on May 26, 1976.
However, the optical waveguide disclosed in the abovementioned proposal allows the light to leak in one direction so that the transmission loss is great and a material having optical anisotropy must be employed. Accordingly, this proposal is not free from the drawbacks of large transmission loss, extremely unstable temperature characteristics and severe restrictions imposed on its production.
Meanwhile, it is known theoretically that in order to maintain the polarization plane in the single mode transmission optical fiber, it is effective to increase the difference between the propagation constants of the two orthogonal polarizations (hereinafter referred to as ".DELTA..beta.") lest rotation of the polarization plane occur inside the optical fiber ("Polarization characteristic of non-circular core single-mode fibers", V. Ramaswamy, W. G. French and R. D. Standley, Applied Optics, Vol. 17, No. 18, 1978).
Furthermore, there is known an optical fiber which controls the concentration of a dopant contained in a cladding of the optical fiber consisting of a core, said cladding and a jacketing and utilizes the difference of birefringence occurring due to the difference of thermal expansion of the materials forming the layers, in order to increase the difference in the propagation constants .DELTA..beta. in the directions of two axes orthogonal to each other (ELECTRONIC LETTERS, I. P. Kaminow, J. R. Simpson, H. M. Presby, J. B. Maccheshey, "Strain birefringence in single-polarization germanosilicate optical fibers", Electronic Letters, Vol. 15, No. 21, pp. 677-679, and Japanese Patent Laid-Open No. 130044/1979 entitled "Optical Waveguide and Method of Producing the Same", laid-open on Oct. 9, 1979.)
Though the optical waveguide disclosed in this proposal is effective, the following problems are encountered in producing the device using a circular waveguide. First, it is difficult to make birefringence at least 10.sup.-5. Since the cladding layer is shaped in an oval form, specific cutting work must be applied to a preform (a glass rod before the production of a thin, final optical fiber by wire drawing) at a certain step of the production process. Furthermore, the core diameter must be reduced markedly down to 1 .mu.m or below. Finally, the difference in the propagation constants .DELTA..beta., that is necessary for the practical use, cannot be obtained.